1. Field of the Invention
This invention relates to oscillators, and more particularly, to a FET transistor voltage-controlled oscillator.
2. Description of Related Art
A metal-oxide-semiconductor field-effect transistor (MOSFET) is a type of FET widely applied in electronic circuits. Typically, a MOSFET comprises three terminals, a gate G, a source S, and a drain D, wherein the gate is formed by stacking a metal layer, an oxide layer and a semiconductor layer sequentially and has a structure similar to a capacitor.
The conduction efficiency of the FET relates to the capacitance of the gate oxide layer of the FET. With the rapid development of modern semiconductor technology, the size of a FET has become smaller and smaller, and the gate oxide layer of a FET has become smaller and thinner. Further, because conducted electrons move along a channel at an interface between the oxide layer and the semiconductor bulk when the FET is turned on, the interface between the oxide layer and the semiconductor bulk has to be fabricated as smooth as possible. Moreover, since the substrate of a semiconductor has low resistance, parasitic capacitance is exhibited in the smooth interface, the low-resistance substrate and around the metal conductors connecting these transistors during operation, affecting the working efficiency of the FET.
This so-called parasitic capacitance is a slight capacitance not designed in on purpose but existing between the conductive traces and the silicon substrate of an integrated circuit. Under a low oscillation frequency condition, this parasitic capacitance effect is not significant and can be neglected. However, the higher the oscillation frequency is, the more significant the parasitic capacitance effect becomes and, thus, can no longer be neglected during design.
Referring to FIG. 1, a conventional inductor-capacitor tank transistor voltage-controlled circuit is shown. The inductor-capacitor tank transistor voltage-controlled circuit comprises a pair of crossed-coupled transistors, a pair of inductors L and a pair of varactors C. In operation, an external voltage source VDD supplies power for the switching mechanism of the gates of the crossed-coupled transistors and provides a working voltage to the transistor voltage-controlled circuit, the inductors L and the varactors C of the inductor-capacitor tank transistor voltage-controlled circuit to determine the oscillation frequency of the circuit.
However, as the oscillation frequency becomes higher and higher, the parasitic capacitance effect is too significant to be neglected. It can be viewed from node 11 or node 12, as shown in FIG. 1, that the conventional inductor-capacitor tank transistor voltage-controlled circuit has a significant parasitic capacitance effect. Since the working frequency of an LC oscillator is inversely proportional to the square root of the total capacitance of the LC oscillator, i.e.,
      f    =          1              2        ⁢        π        ⁢                  LC                      ,the actual working frequency of the inductor-capacitor tank transistor voltage-controlled circuit is reduced by about 30%˜40% due to the parasitic capacitance effect.
Therefore, a definite need exists in the art to provide a voltage-controlled oscillator that can reduce the parasitic capacitance effect when operating at high frequency to increase the working frequency of the voltage-controlled circuit.